The D2 standard is a popular format for digitising NTSC and PAL composite video signals. The analog composite video signals are sampled at four times the chrominance sub-carrier frequency, accurately in phase with the I and Q (or u and V in Pal) colour difference signals. In accordance with this standard, the analog line sychronization and color burst signals are removed (and regenerated during digital to analog conversion) therefore during the sampling procedure, it is important to sample the signals at a specified phase with respect to the color sub-carrier. Thus, to effect sampling, the unmodulated chroma sub-carrier signal must be accurately regenerated from the color burst signal.
A known analog phase locked loop for regenerating the un-modulated chroma sub-carrier is shown in FIG. 1. The circuit includes a voltage controlled oscillator 15 which, in addition to generating the sub-carrier signal, also generates a sampling signal at four times the frequency of the sub-carrier. The synthesised sub-carrier signal from the VCO 15 is supplied to a multiplier 16 which receives at its second input, the colour burst signal via a band pass filter 17. Multiplier 16 produces sum and difference signals and the difference signal is supplied to a loop filter 18, via a low pass filter 19 and a gate 20. The gate 20 ensures that only the color burst is allowed to enter the loop and is placed in its open position at all other times, while the loop filter 18 controls the gain of the circuit. Thus, the filtered difference signal, representing the phase error, is supplied to the VCO 15 which in turn provides its output to the multiplier 16, thereby completing the loop.
The chroma sub-carrier may be representated on a phasor diagram, as shown in FIG. 2A. In the phasor of FIG. 2A, the amplitude of the red minus luma (R-Y) signal may be represented by displacement along the positive y axis and the magnitude of the blue minus luma (B-Y) signal may be similarly represented by displacement along the positive x axis. In accordance with convention, the negative B-Y axis is taken to be the zero phase reference, with phase displacements measure clockwise from this reference, putting the R-Y signal at a phase of 90 degrees.
In the circuit shown in FIG. 1, the analog phase locked loop is responsible for generating the sampling signal and any changes to the operating characteristics of the analog components making up said circuit may result in sample phase errors. It is, therefore, advantageous to calculate sample phase after digitisation of the signal, so that the sample phase calculations may be performed after digitisation, wherein the reference burst signal, as well as the modulated picture signal, is also sampled by the analog to digital convertor.
In known phase locking apparatus is shown in FIG. 3, in which substantial portions of the circuit are implemented digitally. The incoming color burst is converted into a sequence of digital samples by an analog to digital converter 31, at four times the frequency of the colour sub-carrier. The sampling signal is produced by a voltage controlled oscillator 32 and it should be noted that the loop frequency of said oscillator is four times the loop frequency of the oscillator 15 in FIG. 1. In addition ADC 31 is also used for sampling the modulated picture data to produce the D2 image samples.
The colour burst signal is transmitted at the reference zero phase as shogun in FIG. 2A and, when locked, sample clocks are produced at zero 90, 180 and 270 degrees per cycle. As shown in FIG. 2B, the zero degrees and the 180 degrees samples should have values of zero when the VCO 32 locks onto the phase of the sub-carrier, irrespective of the amplitude of the burst signal. Thus, in the circuit shown in FIG. 3, the non-zero samples at 90 degrees and 270 degrees may be ignored, for phase locking purposes.
The samples at zero and 180 degrees will have non-zero values when the output signal produced by the VCO 32 is out of phase with the incoming color burst. If the zero degrees sample, say, is positive, due to the sampling frequency lagging the burst signal, the 180 degree sample will be negative. The samples at zero and 180 degrees are therefore supplied to an alternating inverter 33, which inverts the 180 degree samples, such that all of the samples which should occur at the zero crossings have the same polarity. This signal could be supplied directly to the VCO 32 as a control signal, however, it is preferable for the signals to be accumulated throughout the duration of the burst period in an accumulator 35, before being supplied to the VCO 32 via a digital to analogue converter 36 and a loop filter 37, wherein said loop filter 37 is substantially similar to loop filter 18 in FIG. 1. Thus, whenever the output from the digital to analogue converter is non-zero, a correcting error signal is supplied to the VCO 32, thereby bringing the sampling frequency into phase with the burst signal.
In the generation of analog composite video signals, the modulated chroma sub-carrier and the reference burst signal must, to some extent, be processed along different channel paths before being multiplexed for transmission. This may introduce phase errors between the modulated carrier and the burst reference, resulting in hue errors. It is therefore common practice to provide some form of hue control, so as to compensate for the phase error and thereby ensuring that the D2 samples are taken at the correct positions.
The relationship between sample phase and colour burst is further complicated in the NTSC system by the fact that the polar representations of the I and Q axes are displaced from that of the colour burst by 57 degrees. Thus, both the I and Q sample points do not occur at zero crossings of the burst signal, such that the sum of said samples during the burst period is non-zero when the sample phase is correctly locked. This situation, of samples being taken at non-zero crossings with respect to the burst signal, also occurs if the sampling phase must be adjusted to provide hue compensation. Thus, the phase locked loop shown in FIG. 3 operates perfectly well when the samples are required to be in phase with the color burst signal (in PAL systems without hue control) but it will not operate where it is necessary to introduce a phase displacement.
Referring to the circuit shown in FIG. 3, it would be posssible to introduce an offset value, so as to compensate for a phase displacement. For example, accumulator 35 may be loaded with an offset value prior to or subsequent to the accumulation of sample values, such that lock is obtained for sample points displaced along the x axis of the waveform shown in FIG. 2B, away from the zero crossings. However, the burst signal is a purely analogue waveform and may have, within reasonable limits, any absolute magnitude. Thus, as can be appreciated with reference to FIG. 2B, the absolute value of any sample taken at positions other than the zero crossings, will be dependant, not only upon sample phase, but upon the peak to peak magnitude of the chroma burst signal. Thus, any offset value will be dependant, not only upon the phase displacement required but also upon the absolute value of the chroma burst amplitude.
The I and Q axes are displaced from the R-Y and B-Y axes in order to reduce transmission bandwidth. The I axis occupies a position relating to colors to which the eye is most sensitive and conveys frequencies up to 1.5 megahertz. However, the eye is least sensitive to colours that lie around the Q axis, therefore this signal is allocated a reduced bandwidth of 0.5 megahertz above the frequency color of the sub-carrier. Thus, the I signal is displaced from the colour burst signal by a phase of 57 degrees, therefore the technique of relying upon the zero crossings, as illustrated in FIG. 3, cannot be used to achieve sampling at the I and Q positions, unless a phase error is introduced, resulting in the inherent problems of placing reliance upon the absolute amplitude of the color burst signal.
Referring to FIG. 2A, the color burst signal in the NTSC system has zero phase, however, from this a sampling waveform must be produced which is in phase with the I and Q axes, thereby ensuring that the chroma signal is sampled in accordance with the D2 standard.
The Q signal may be considered as being displaced from the positive B-Y axis by a fixed phase displacement angle, P degrees. Similarly, the I axis is displaced by P degrees from the positive R-Y axis. In accordance with the D2 standard, the picture data and therefore the incoming burst, must be sampled at the I and Q positions. However, accurate sample phase adjustment can only be achieved by accumulating values which should be zero, that is along the B-Y axis, thus the B-Y values are calculated and accumulated over the burst period, in order to produce an error signal for controlling the VCO.
As shown in FIG. 2A, the sum of the I and Q phasors has a component in the B-Y axis equal to: Q cos P-I sin P, which may be calculated by a circuit of the type shown in FIG. 4. Thus, in FIG. 4, each I sample is multipled by the sine of the phase error in a multiplier 41, each Q sample is multipled by the cosine of the phase error in a multiplier of 32 and the I component is subtracted from the Q component to produce the B-Y value. The B-Y signals may now be used in a circuit similar to that shown in FIG. 3, in which said signals would be applied to the accumulator 35, given that said signals should have a value of zero when the sampling signal generated by the VCO 32 has the correct phase relationship with the incoming burst signal.
Such an arrangement may also be used for providing a hue control, in which P consists of the selected hue phase adjustment added to the fixed phase adjustment signal of 33 degrees, in order to achieve correct I and Q sampling.
A major problem with the circuit implementation shown in FIG. 4 is that real time multipliers 41 and 42 add significantly to the overall cost of the circuit. Furthermore, although multipliers are commercially available which could operate at video rate, in other applications the speed of operation of the multipliers could become a limiting factor, thereby reducing transmission rate.